Handle AlphaData in figure

 - If there is an Alpha Channel, always produce a TrueColor image
 - Handle all `AlphaDataMapping` modes:
   - `none` (nothing special)
   - `scaled`: map to the same range. According to MATLAB, Inf in `Alim`
     is possible. Handling that case by using the maximum encountered.
   - `direct`: use the AlphaMap in the figure.
Merge branch 'alphaChannel'